1. Field of the Invention
The present invention relates to a Liquid Crystal Display (LCD) device and a driving method thereof, and more particularly, to an LCD device including a liquid crystal panel with a built-in touch panel and a driving method thereof.
2. Discussion of the Related Art
LCD devices adjust the light transmittance of liquid crystal with an electric field to display an image. To this end, the LCD devices include a liquid crystal panel in which a plurality of pixels are arranged in a matrix type, and a driver for driving the liquid crystal panel.
Methods, which input a control signal into an electronic product with LCD devices mounted thereon, include a method using a touch panel and a method using buttons. Recently, the method using the touch panel is widely used.
Touch panels may be formed in various types depending on an arrangement position. That is, the touch panels may be formed in an on-cell type in which the touch panel is adhered to a top of a color filter substrate, an in-cell type in which two electrodes configuring the touch panel are formed in a thin film transistor (TFT) substrate configuring a liquid crystal panel, or a hybrid type in which one of two electrodes configuring the touch panel is formed in the TFT substrate of the liquid crystal panel and the other is formed at the top of the color filter substrate. Here, a liquid crystal panel including the in-cell type touch panel or the hybrid type touch panel is referred to as an in-cell touch panel.
FIG. 1 is an exemplary diagram illustrating an equivalent circuit of a touch panel applied to a related art in-cell touch panel, and FIG. 2 is an exemplary diagram illustrating capacitance generated between a touch electrode and a gate line in the related art in-cell touch panel.
An equivalent circuit of a touch panel 20 applied to the related art in-cell touch panel, as shown in FIG. 1, may include a driving signal resistor RTX connected to a touch driver 10 that applies a driving pulse (Tx signal) to the touch panel 20, a sensing signal resistor R connected to a sensing signal receiver 30, and a plurality of capacitance CMutual, CTX and CRX that are connected in parallel or series.
In addition to the above-described capacitances, as shown in FIGS. 1 and 2, the related art touch panel 20 may further include gate line capacitances CGL1 and CGL2 that are generated between a gate line (GL) 41 and a driving electrode 21 or a receiving electrode 22 configuring a touch electrode. The gate line capacitances CGL1 and CGL2 may cause noise to the driving electrode 21 and the receiving electrode 22 to deteriorate a touch determination function of a touch sensing unit including the sensing signal receiver 30. A mathematical analysis on this will be provided below.
A portion (a) of Equation (1) is an equation for determining whether there is a touch with no consideration of a gate line capacitance, and a portion (b) of Equation (1) is an equation for determining whether there is a touch in consideration of the gate line capacitance.
                                          (            a            )                    ⁢                                          ⁢                                    Δ              ⁢                                                          ⁢                              C                m                                                    C              i                                      ⁢                                  ⁢                              (            b            )                    ⁢                                          ⁢                                    Δ              ⁢                                                          ⁢                              C                m                                                                    C                i                            +                              C                GL                                                                        (        1        )            where ΔCm denotes an amount of change in a capacitance generated by a touch, CGL denotes a gate line capacitance generated between the gate line and the touch electrodes, and Ci denotes an initial capacitance generated in the touch panel.
In Equation (1), it can be seen that when the amount of changed capacitance (ΔCm) in the portion (a) is equal to that in the portion (b), a value of the amount of changed capacitance (ΔCm) divided by the initial capacitance (Ci) and the gate line capacitance (CGL) is less than a value of the amount of changed capacitance divided by the initial capacitance (Ci). Here, as the gate line capacitance (CGL) increases, an amount of change in a signal detected by the sensing signal receiver 30 becomes less, and thus, a touch sensitivity of the touch sensing unit including the sensing signal receiver 30 becomes lower.
Generally, in in-cell touch panel 40, the driving electrode 21 and the receiving electrode 22 share the same gate line 41. Therefore, the related art in-cell touch panel has the following limitations.
First, when a driving pulse is generated, there is extra charge transferred through the gate line 41 shared by the driving electrode 21 and the receiving electrode 22, and the size of the extra charge is changed according to the resistance of the gate line that maintains a constant voltage.
That is, as the resistance of the gate line becomes lower, characteristic in which the gate line intends to maintain a specific voltage increases. Therefore, even though there is a coupling capacitance (gate line capacitance (CGL)) between the gate line and the driving electrode or the receiving electrode, the touch sensitivity is enhanced because the extra charge transferred from the driving electrode to the receiving electrode through the gate line is reduced.
However, in the in-cell touch panel of the related art, a separate configuration and method for decreasing the resistance of the gate line are not proposed.
Second, in a Gate-In Panel (GIP) type LCD device in which a gate driver is formed in the in-cell touch panel, generally, while a scan signal (pull-up signal) is not inputted to the gate line during one frame, a pull-down signal for turning off switching transistors formed in the gate line is continuously inputted to the gate line. In this case, if one pull-down transistor continuously inputs the pull-down signal to the gate line, the performance of the pull-down transistor is reduced.
Therefore, the GIP type LCD device alternately drives two pull-down transistors to input the pull-down signal into the gate line, in each frame.
However, since different pull-down transistors input the pull-down signal to the gate line in each frame, due to a characteristic difference between pull-down transistors, the characteristic of a resistance for maintaining the gate line is changed whenever an alternation period is changed. The change in the resistance characteristic of the gate line changes the size of the extra charge transferred from the driving electrode to the receiving electrode through the gate line, and thus, changes touch characteristic, causing the reduction in a function of detecting whether there is a touch.